The present invention relates to a voltage supply for an electric filter.
Electric filters usually operate with a high dc voltage obtained by rectification of a voltage supplied by an ac network (see, for example, "Siemens-Zeitschrift," 1971, No. 9, pages 567 to 572). It is known to superimpose on this high dc voltage supplied to the filter a pulsed voltage which is dependent on the operating state of the filter and can be generated, for example, in response to a short circuit in the filter, as disclosed for example in DE-OS 26 08 436 and DE-OS 30 27 172.
Pulsed voltage sources for providing the pulsed voltage are also known, according to which a thyristor and a diode are connected in series with a dc voltage source and a transformer coupled to the filter. The resonant circuit formed by the transformer and the pulsed voltage source is triggered each time the thyristor is fired, as described for example in DE-OS 26 08 436, to provide an oscillation of the resonant circuit which is delivered to the filter as a pulsed voltage via the transformer.
A breakdown of the electric filter resulting in a short circuit usually occurs at the time that the maximum voltage is applied to the filter, i.e. during the period the diode carries the pulsed current (oscillation), or shortly thereafter. Due to the filter short circuit, the resonant circuit oscillation is abruptly damped, i.e. the diode is blocked. Thereby, the maximum dc voltage is reapplied to the thyristor. If the time between cut-off of the thyristor by the zero crossing of the current and transfer of current through the diode is very short, i.e. shorter than the recovery time of the thyristor, the thyristor can be fired by the reapplied maximum dc voltage without a firing pulse. Since this firing of the thyristor proceeds relatively slowly, the thyristor is subjected to high thermal stress during this period and may be destroyed.